Electrostatic grounding probe for use in explosive atmospheres

ABSTRACT

A grounding probe having a stationary contact connected to ground through a high resistance. A movable contact is normally biased away from the stationary contact but can be moved adjacent the stationary contact by pressing it against the object to be grounded. The area where stationary and movable contacts come together is enclosed so that any spark is isolated from the atmosphere.

United States Patent 1 13,577,20s

[72] Inventor JohnT.Petrick [50] FieldofSearch 174/5,5.l, Fredericksburg, Va. 5.2, 6; 200/ 144 (APR), (inquired), 51.09

21 AppLNo. 838,923

221 Filed July 3,1969 1 References Cited 45 Patented May 4,1971 UNITED STATES PATENTS I731 Assignee Thevnited stateslfAmeric-a asrrepresemed' 2,020,402 11/1935 Edwards et al. 200/51.09x bythesecretaryoflheNavy 2,753,491 7/1956 Legge l74/6X Primary Examiner-Lee T. Hix Attorneys-Edgar J. Brower and Thomas 0. Watson, Jr.

[54] ELECTROSTATIC GROUNDING PROBE FOR USE ABSTRACT: A grounding probe having a stationary contact gp connected to ground through a high resistance. A movable alms awmg contact is normally biased away from the stationary contact [52] US. Cl 317/2, but can be moved adjacent the stationary contact by pressing 174/6, 200/5 L09, 200/144AP it against the object to be grounded. The area where stationary [51] Int. Cl H0lr 3/06, and movable contacts come together is enclosed so that any H05f spark is isolated from the atmosphere.

ELECTROSTATIC GROUNDING PROBE FOR USE IN EXPLOSIVE ATMOSPHERES GOVERNMENT INTEREST IN THE INVENTION 7 BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates generally to electrostatic grounding devices and in particular to electrostatic grounding devices in which anysparks created are isolated from the surrounding atmosphcre.

2. Description of the Prior Art Prior art grounding devices and particularly those devices used to ground ammunition were usually unsafe when used in explosive atmospheres or around electroexplosive devices. A simple ground wire connection to ammunition results in a-high energy discharge which may ignite dust or explosive vapors in the atmosphere. High current flow through this ground wire may also induce electrical currents in electroexplosive devices nearby thereby detonating' them. Furthermore, since relatively high currents have to be carried by the ground wire. prior art grounding devices include bulky wires and bulky ground connections making them cumbersome to handle.

SUMMARY OF THE INVENTION Applicant has devised a device to overcome the disadvantages of the prior art by slowly draining the electric charge from an electrostatically chargedobject. such as ammunition, and preventing an energetic spark from occuring in a potentially dangerous atmosphere. A movable contact ispressed against the charged object thereby causing closure of a spark gap between the movable contact and a ground contact. The ground contact is grounded through a series of high resistances, and the area of the spark gap is enclosed from the surrounding atmosphere. I

A primary object of this inventionis to provide a device for grounding an electrostat ally charged object which isolates any spark from the surrou fiding atmosphere;

,It is another object of this invention to provide a device which drains the charge relatively slowly from an electrically charged object.

It is a further object of this invention to provide a grounding probe in which the current carried by the grounding wire is relatively small.

It is still a further object of this invention to provide a grounding probe whichcan be used without the danger of shock to the user.

Another object of this invention is to provide a grounding probe which can be activated by pressing it against a charged object.

Other objects, advantages and novel features'of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. v

BRIEF DESCRIPTION OF THE DRAWINGS l FIG. I shows in cross section. the electrostatically ground-1 DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of applicant's grounding probe is 1 the probe is ready for u. again.

the forward end of casing'l4 is an aperture 16 and a wall 2] having an aperture therethrough aligned with aperture 16. A movable probe I5 is mounted in aperture 16 arid the aligned aperture in wall 2I for sliding movement lengthwise of the probe. Ahelical spring I9 having one end 20 secured by conventional means to probe I5 and its other end 22 inabutment with a wall 21 is provided to normally bias movable probe I5 to a position in which it is extended from casing I4. Rearwardly of wall 21 is another wall 26 having mounted therein a stationary probe 25. The area between stationary probe 25 and end 24 of movable probe I5 is a spark gap which, as can be seen, is in an enclosed spark chamber 27. An O-ring 18 or other sealing device makes a seal with movable probe 15 to prevent dust or explosive vapors from entering spark chamber 27 through aperture .16. Stationary probe 25 is grounded through a series of resistors 28, a light grounding wire 29 and a light ground connection 30. The total resistance provided by resistors 28 depends on the size of the charge to be discharged by the probeQGenerally the total resistance should be greater than l0 megohms. In practice a resistance of I32 megohms has been used tosafel y discharge potential up to 30,000 volts.

As can be seen, spring I9 normally biases probe I5 so that spark gap 27 is in an open condition. To operate applicant's grounding probe casing I4 is grasped and movable probe 15 is pressed against'an electrically charged object such as ammunition. This may cause a small spark between probe I5 and the I object; however. the magnitude of this spark is insufficient to ignite a potentially dangerous surrounding atmosphere under normal operating conditions because of the small capacitance of probe IS. The device is then pushed toward the object, thus compressing the retainer spring I9 and causing spark gap 27 to close. As the gap closes there will be a particular distance at which the air within the gap undergoes electrical breakdown.

' This particular distance will depend upon the charge being grounded. When the breakdown occurs a spark will result between end 24 of probe I5 and probe 25. The current flow during breakdown is limited by resistors 28 thereby preventing I a powerful spark front-occurring in spark chamber 27. When T the gap is fully closed, as shown in FIG. 2, the ammunition is completely discharged. When the probe is removed from the dbject spring Iiilreturns probe I5 to its extended position and l The original spacing of end 24 of probe 15 and probe 25 will detemiine the operating range of the probe. It has been found that a 1 inch gap allows proper operation at electric potentials up to 30,000 volts. For higher voltages a larger gap would have to be used. 1

Use of a high resistance 28, besides limiting the power discharged in the spark gap 27, also reduces the current flow in ground wire 29. This lessens the likelihood that currents will be induced innearby electroexplosives by the current in shown in FIG. I, generally designated by reference numeralground wire 29. This reduced current flow also permits use of a light ground wire and ground connectors to increase portability and ease of handling of the grounding probe. Furthermore. since grounding wire 29 carries a small current, hazards from accidental contact with power lines or electronic equipment are reduced.

FIG. 3 shows an alternate construction for applicants grounding probe. In this embodiment probe 15 is f xed to a flexible diaphragm 3I. When the probe is pr ssed against the charged object diaphragm 31 is flexed inward thereby closing the spark gap 27.

FIG. 4 shows another alternate embodiment of applicant's invention. In this embodiment movable probe 15 is fixed to a bellows 32. The bellows is normally in an extended position, however it can. be compressed by pressing probe I5 against an object to be grounded to close spark gap 27.

Obviously many modifications are possible in view of the above teachings. The probe IS in the preferred embodiment should be a fair conductor, rigid, and have end 24 shaped into a hemisphere of a diameter preferably not less than one-tenth of an inch. In practice it has been made of brass having a steel ball at end 24. However other possibilities are clearly available. Probe body 14 can be constructed of any rigid material of high dielectric and mechanical strength. Plastics such as Lucite." Plexiglass or Lexan may be used. O-ring 18. used to seal the spark chamber in the preferred embodiment from dust or vapors, is only one of several possible sealing means. Also spark chamber 27 may contain or be pressurized with dry nitrogen or other insulating vapor thus increasing the potential necessary for breakdown of the spark gap. Resistors 28 may be of any construction.

I claim:

l. A grounding device for use in explosive atmospheres comprising:

a housing made from insulating material;

a spark chamber within said housing a first contact mounted within said spark chamber;

a movable second contact mounted within said spark chamber and extending outwardly from said housing;

biasing means retaining said second contact in a position normally spaced from said first contact;

means for grounding said first contact;

said grounding means including high resistance means enclosed in said housing and separated from the spark chamber by a partition; and

sealing means surrounding the outwardly extending portion a of said movable contact at the front end of saidhousing to 'prevent explosive vapors from entering said spark chamber.

2. The grounding device of claim 1 wherein said biasing means comprisesa spring.

3. The grounding device of claim 2 wherein said sealing means comprises an O-ring.

4. The grounding device of claim 1 wherein said biasing means comprises a diaphragm attached to the front end of said housing.

5. The grounding device of claim'4 wherein saidv biasing means comprises a bellows attached to the front end of said housing. 7

6. The grounding device of claim 1 wherein said spark chamber contains an insulating vapor under pressure. 

2. The grounding device of claim 1 wherein said biasing means comprises a spring.
 3. The grounding device of claim 2 wherein said sealing means comprises an O-ring.
 4. The grounding device of claim 1 wherein said biasing means comprises a diaphragm attached to the front end of said housing.
 5. The grounding device of claim 4 wherein said biasing means comprises a bellows attached to the front end of said housing.
 6. The grounding device of claim 1 wherein said spark chamber contains an insulating vapor under pressure. 